Put sugar in a 9-inch cake pan or pie plate, set on the center rack of oven and bake until the sugar is just caramel colored 15 to 25 minutes.  Watch closely.  Swirl to cover the bottom with the caramel.

In a large bowl, whisk together the brown sugar, cinnamon, ginger, nutmeg, and salt.  Stir in pumpkin.  In a medium bowl, whisk together the half-and-half, eggs, and vanilla.  Thoroughly blend the egg mixture into the pumpkin.

Set the pie plate in a large roasting pan, and pour the custard over the caramel.  Pour enough hot water into the roasting pan to reach halfway up the sides of the plate.  Bake until the custard is set, about 1 hour or more.  Cool and chill in the refrigerator.  Run a knife around the outside edge of the flan and invert onto a rimmed plate.  Serve wedges with a dollop of whipped cream, if desired.  Serves 8.